Nuclear density gauges are often used to measure liquids (the term liquid, when used in this application for patent, includes slurries) in a vessel at places such as oil refineries. As illustrated in prior art FIG. 1, a nuclear density gauge 10P has two parts that are separated with the liquid to be measured located between them. The first part is a sealed source of radiation 14P, commonly referred to as a “source.” The second part is a detector 16P, for example, a scintillation detector. When radiation leaves the source, the amount reaching the detector aligned with the source decreases as the distance between them increases, even if there is only a vacuum between them. When passing through a liquid 18P in a vessel 20P, the mass of the liquid absorbs some of the radiation. In addition to the liquid, if the source, the detector, or both, are located outside the vessel, the radiation must also pass through at least one of the vessel walls 22P. Since the absorption of the radiation by the walls 22P is constant, and the distance between a source 14P and a detector 12P is constant, the amount of radiation reaching the detector is indicative of the density of the liquid 18P it passes through. As the density of the liquid changes, the amount of radiation reaching the detector changes. The greater the density of the liquid, the less radiation reaches the detector. The detector typically provides density measurement in the form of a current output.
It should also be noted, that to keep a detector cool, and make it easier to maintain, detectors 12P are almost always located outside the vessel 20P. The source 14P, however, is durably packaged, and may be placed inside the vessel to be operated in extremes of pressure and temperature safely, using insertion tubes.
In installations where the vessel walls 22P are thin, and the distances across the vessel 20P are small, for example less than two to three feet, a source 14P can be on one side of the vessel, and the detector on the other side as schematically shown in prior art FIG. 1. Such an arrangement is also often used to measure density inside a pipe. Then radiation passes through a first vessel wall, the entire length of liquid, and a second vessel wall before reaching the detector. This works best when the vessel walls are thin and the path through the liquid is short. For liquid density measurements, for example, this arrangement is typically used when the path through the liquid is not greater than two or three feet. For thick walls or long paths, more radiation is absorbed, decreasing measurement sensitivity. Larger sources may be used, but compensating with ever larger sources of radiation becomes impractical.
For larger vessel sizes or greater wall thicknesses, there are ways to decrease the material that must be penetrated. For larger vessels or thicker walls, what is known as an “internal solution” can be used as shown in prior art FIG. 2. An internal solution entails placing the source 14P inside the vessel, and the detector 16P, outside the vessel. This decreases the distance of liquid 18P the radiation passes through, and decreases the number of vessel walls 22P from two to one.
There are also limits on the size of vessel for which the configuration of FIG. 2 is a desired solution. Distance, steel thickness, and liquid density all play a role. A general rule is that 4 inches of water, and ½ inch of steel, have about the same affect on the radiation. However, liquids being measured are not usually only water. As a general rule, if a distance is greater than 2 feet, or steel thickness greater than 2 inches, configurations other than FIG. 1 or FIG. 2 are necessary.
Another internal solution is schematically illustrated in prior art FIG. 3. In this Figure, the vessel wall 22P is thick. At the top of the figure, the wall is locally thinned at 24P. One way of doing this is to drill a hole 26P starting at the exterior, but stopping before reaching the interior. This hole 26P is often called a “detector well” in the industry. The area around the hole 26P is reinforced (not shown) as necessary, for example by welding on additional metal plating. A source 14P is placed in line with the detector. This is done by drilling a hole 28P through the vessel wall and welding in place a nozzle that can accept a sealed tube 30P that can contain the source. The area around the hole 28P is reinforced (not shown) as necessary, for example by welding on additional metal plating. This sealed tube 30P is known as a “source well” and is often referred to as a “dry well.” Radiation leaving this source will pass through liquid near the vessel wall and a thin wall 24P to reach the detector.
Another type of detector well 32P is shown in FIG. 3 at the left side of the figure. This detector well fully penetrates the vessel wall through a nozzle with a welded in place sealed tube positioned interior to a detector. Another source well 29P is used to place a source in line with the detector well 32P. Radiation leaving this source will pass through a liquid path close to the center of the vessel, through a relatively thin well wall 33P, through a distance of air 34P, and reach the detector.
The prior art of FIGS. 1, 2, and 3 all have disadvantages, some of which are explained in the following:
As already stated, the configuration of FIG. 1 is limited to smaller vessels and thinner vessel walls.
In FIG. 2, alignment of the sources with the detectors can be difficult to achieve and maintain. It also does not accommodate thick walls. In addition, as vessel pressure and temperature change the vessel expands and the relative position of the sources and detectors changes. Large vessels intended for high pressure and high temperature operation may change length as much as 10 inches when going from ambient to operating conditions. Further, the source well inside the tank is subject to buffeting that may move its location relative to the detector. The detector, mounted outside the tank, may be inadvertently moved relative to the source. In short, the detector and the source are not coupled to one another, and therefore they are subject to different conditions that can move them out of alignment, creating significant measurement errors.
In FIG. 3, the internal solutions, each of which require two holes in the vessel, are undesirable for at least three reasons. First, especially when dealing with high temperatures and pressures, it is preferred that vessel walls not be modified and be left in their full un-cut state, to maintain maximum integrity, without the addition of reinforcements. Second, drilling two precisely aligned holes, for example 26P and 28P, through a thick wall is very difficult. Even if aligned correctly at the start of the drilling, the drill can deflect off course while passing through thick metal. And third, even if the source and detector have a known geometric relationship to one another in a cool vessel, this can change as the temperature and pressure in the vessel changes. This may be further exasperated by the reinforcements placed around the holes that may cause non-uniform metal expansion, and thwart attempts to predict movement.
The need for higher temperature and pressure vessels, and their thick walls, is increasing for a variety of reasons, including processes developed by the refining industry for upgrading heavy-oils. An example of this process is EST (Eni Slurry Technology) being developed by Eni corporation of Italy, as at least partly described in US Published Patent application 2006/0163115. Another example is VRSH (Vacuum Resid Slurry Hydrocracking) developed by Chevron Corporation, as partly described in U.S. Pat. Nos. 7,238,273 and 7,214,309.
Thus, there exists the need for a nuclear density gauge configuration for more accurately and easily measuring the density of a liquid in a thick walled vessel at high temperature and pressures. Such a gauge also has advantages in less severe applications.